With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 160 GB per one magnetic disk with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 250 Gbits per one square inch is desired to be achieved.
To attain a high recording density in a magnetic disk for use in an HDD or the like, a magnetic disk of a perpendicular magnetic recording type has been suggested in recent years. In a conventional in-plane magnetic recording type, the axis of easy magnetization of a magnetic recording layer is oriented in a plane direction of a base surface. In the perpendicular magnetic recording type, by contrast, the axis of easy magnetization is adjusted so as to be oriented in a direction perpendicular to the base surface. In the perpendicular magnetic recording type, compared with the in-plane recording type, a thermal fluctuation phenomenon can be more suppressed at the time of high-density recording, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
Conventionally, as a magnetic recording layer, CoCrPt—SiO2 or CoCrPt—TiO2 has been widely used. In Co, a crystal of a hcp structure (a hexagonal close-packed crystal lattice) grows in a columnar shape. And Cr and SiO2 (or TiO2) are subjected to segregation to form a non-magnetic grain boundary. By using such a granular structure, physically independent fine magnetic grains can be easily formed, and a high recording density can be easily attained.
Also, for example, Patent Document 1 discloses a perpendicular magnetic recording medium in which a recording density can be further increased. This includes two magnetic recording layers, thereby improving a coercive force Hc to increase the recording density. However, an oxide contained in each layer is of one type (SiO2), and each layer has a different composition ratio.
In the perpendicular magnetic recording medium, a medium protective layer is provided that protects the surface of the magnetic recording layer so that the surface is not damaged when the magnetic head collides with the perpendicular magnetic recording medium. The medium protective layer forms a high-hardness coating with carbon overcoat (COC), that is, carbon coating. In the medium protective layer, the carbon coating may include hard diamond-like coupling and soft graphite coupling in a mixed manner (for example, Patent Document 2). Also, a technology of manufacturing a diamond-like coupling protective film by CVD (Chemical Vapour Deposition) is also disclosed (for example, Patent Document 3).
On the other hand, to increase reliability of shock resistance, abrasive resistance, corrosion resistance, and others of the perpendicular magnetic recording medium, it is known that increasing a spectral ratio Dh/Gh with a Raman spectrum of the medium protective layer with carbon hydride as a main component formed on the magnetic recording layer is effective. To increase Dh/Gh of the medium protective layer, it is known that heating the disk base having the magnetic recording layer formed thereon at the time of forming the medium protective layer is effective (for example, Patent Document 4).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-155861
Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-11734
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2006-114182
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-149553